Austenitic ductile iron having high notch ductility at low temperature



United States Patent G AUSTENITIC DUCTILE IRON HAVING HIGH NOTCHDUCTILITY AT LOW TEMPERATURE Robert D. Schelleng, North Plainfield, andWilliam K.

Abbott, Bound Brook, N..l., assignors to The International NickelCompany, Inc., New York, N.Y., a corporation of Delaware No Drawing.Filed June 30, 1961, Ser. No. 120,976

Claims. (Cl. 75-123) The present invention relates to austenitic ductilecast irons and, more particularly, to austenitic ductile irons having aspecial composition which are characterized by high impact resistance atthe temperatures employed in cryogenic service.

Cast iron is one of the oldest commercial metals and has many practicaladvantages, including ready castability into intricate shapes,machinability, simple melting practice, low liquid shrink-age with highpercentage yields, low density as compared to non-ferrous metals such asbronzes and economic advantages over many other ferrous and non-ferrousmaterials for cryogenic service. The usual types of cast iron containingflake graphite have very limited ductility and impact strength even atroom temperature and the use of such materials in cryogenic service isunthinkable. The recent advent of ductile cast iron has made avail-ableto the art a material having substantial ductility and impact strengthat room temperature. However, the ferritic types of ductile iron undergoimpact transition at temperatures only slightly below room temperaturewith the result that such materials have severely limited application atthe temperatures encountered in cryogenic service. It is accordinglynecessary to employ an austenitic grade of ductile iron when provisionof a material for cryogenic service is the objective. Even here,however, it is found that the commercially available grades ofaustenitic ductile iron suffer from the disadvantage of inadequateductility and impact resistance when subjected to very low temperaturessuch at the temperature of boiling liquid hydrogen, i.e., 423 F.Although many attempts were made to overcome the foregoing difficulties,none, as far as we are aware, was entirely successful when carried intopractice commercially on an industrial scale.

It has now been discovered that an austenitic ductile iron having aspecial composition has very good ductility and impact resistance atordinary temperaures and retains its impact resistance and ductilitydown to temperatures of interest in cryogenic applications, i.e.,temperatures as low as' about 423 F.

It is an object of the present invention to provide an austeniticductile cast iron composition having improved properties.

A further object of the present invention is to provide an austeniticductile cast iron having improved ductility and impact resistance.

Itis also an object of the present invention to provide an austeniticductile cast iron having high ductility and impact resistance at thevery low temperatures required for cryogenic service.

Another object of the present invention is to provide cast articles madeof a special austenitic ductile cast iron which may be employed at verylow temperatures as are encountered in cryogenic service.

Other objects and advantages will become apparent from the followingdescription.

The present invention contemplates austenitic ductile irons containingabout 20% to about 24% nickel, about 2% to about 3% carbon, about 1% toabout 3% silicon, with the sum of the carbon content plus 0.06 times thenickel content plus 0.2 times the silicon content being not more than4.4, about 3.25% to about 5% manganese, not

more than 0.25% chromium, a small amount, e.g., about 0.04% to about0.12%, of magnesium eiiective to induce the occurrence of spheroidalgraphite in the cast iron, and the balance essentially iron. These castirons are characterized by freedom from carbides and freedom frommartensite even when cooled to temperatures as low as about 423 F. andare further characterized by weldability and good foundingcharacteristics,

The nickel and manganese contents of the alloys are very important andthe contents of these elements must be maintained within the foregoingranges in order to produce the improved results found in alloys Withinthe invention. Thus, the nickel content of the alloy must be at leastabout 20% because lower levels lead to austenite instability andmartensite formation and must not exceed about 24% because no furthergains in properties are evident. In addition, the manganese content mustbe at least about 3.25% because lower levels lead to austeniteinstability and martensite formation and must not exceed about 5%because of carbide formation which causes embrittlement.

The alloys produced in accordance with the invention should besubstantially devoid of copper, and in any event should not contain morethan about 0.25% of copper as an impurity. Since the alloy containsmagnesium, which is a sulfur-avid element, sulfur in the alloy ispresent only in limited amounts, if at all, and usually will not bepresent in amount exceeding 0.02%. Phosphorus, a common impurity in castirons, should not be present in amounts exceeding about 0.10%. Thecarbide-forming elements such as chromium, molybdenum, tungstem andvanadium should be substantially absent from the alloys as their eflFectin the alloys is undesirable. Thus, these elements should not be presentin the alloys in amounts exceeding a total of about 0.25 Otherimpurities such as antimony, cerium, bismuth and lead should be keptbelow 0.003% total and titanium preferably should be less than 0.02% asthese impurities have a deleterious effect upon the spheroidal graphitestructure.

It is important that the graphite structure in the alloys contemplatedin accordance with this invention consist of spheroids. The presence ofchunky and flake graphite in the alloys in any quantity should beavoided as otherwise the strength, ductility and impact resistance ofthe alloys are all detrimentally affected. In order to insure thepresence of spheroidal graphite structures in the alloys of theinvention, the composition must be maintained within the foregoinglimits to insure that the total graphitizing power of a melt from whichcastings are to be made is controlled such that the alloy freezes in theiron graphite system, e.g., free of carbide.

The alloy may be prepared in the usual melting equipment used forproducing cast iron, e. g., the cupola furnace, the electric arcfurnace, the induction furnace, the air furnace. The ingredientsrequired for producing the alloy are melted together, brought to propertemperature, e.g., about 2750 F. to about 2850 F., at which point themagnesium is added, e.g., as a nickel-magnesium alloy containing about12% to about 30% magnesium with the balance essentially nickel, the meltis then inoculated with a graphitizing inoculant and metal from the meltis then cast. The final graphitizing inoculation with about 0.25% toabout 1% of silicon, e.g., about 0.5% silicon, is a very important stepand is required in order to produce good spheroidal graphite structures.Usually, ferrosilicon, a commercial graphitizing alloy containing about70% to about silicon, about 0.5 to about 1.0% calcium and the balanceessentially iron, is employed for the final graphitizing addition.However, other silicon-containing agents or alloys such asnickel-silicon alloys, or nickel silicide, calcium-silicon alloys orcalcium silicide, silicon metal, and various proprietary inoculatingalloys commonly used for reducing dendriticism and reducing chill infoundry gray cast iron may be employed for this purpose. While it hasbeen pointed out hereinbefore that the magnesium may advantageously beintroduced into the melt by adding magnesium to the melt as anickel-magnesium alloy, other well known magnesium containing additionalloys may be employed for this purpose.

Alloys produced in accordance with the present invention have very goodproperties at atmospheric temperatures and at very low temperatures,even in the ascast condition. However, it is advantageous to subjectcastings made of the alloy to a normalizing treatment comprising heatingto the temperature range of about 1600 F. to about 1900 F., e.g., 1700F. to 1800 F., for at least about one hour per inch of section followedby air cooling. The normalizing treatment retains carbon in solution inthe austenite and increases the austenite stability preventingmartensite formation at low temperatures.

For the purpose of giving those skilled in the art a betterunderstanding of the invention, the following illustrative examples aregiven:

EXAMPLE I Two melts of the alloy contemplated in accordance with theinvention were made by melting charges of pig iron, ingot iron,electrolytic nickel and ferro alloys in an induction furnace. Eachcharge was brought to a temperature of about 2900 F., cooled to 2800 F.and was then treated with an addition of about 1% of an addition alloycontaining about magnesium, about 2% carbon and the balance essentiallynickel after which each charge was inoculated with about 0.5% silicon asa calcium-bearing ferrosilicon containing about 85% silicon. Metal fromeach of the thus-treated charges was cast into castings, includingl-inch, 2-inch and 3-inch thick plates. It was found that themicrostructures of the two alloys were devoid of carbide and ofmartensite and contained good spheroidal graphite. Metal from each ofthe melts was analyzed with the results set forth in the following TableI:

Table I Alloy TC, S Mn, Ni, Cr, Mg, P,

No. Percent Percent Percent Percent Percent Percent Percent Metal fromthese alloys was subjected to tensile tests with the results set forthin the following Table II:

In addition, metal from these alloys was subjected to Charpy V-notchimpact tests with the following results:

The specimens employed for the foregoing impact tests measured 0.394inch by 0.394 inch on each side, with a 45 notch 0.079 inch deep and hada radius at the bottom thereof 0.010 inch in length. The foregoing datareported is an average of three tests with the exception of the firstvalue reported for Alloy No. 1 which represents an average of six testsand of the last value reported for Alloy No. 1 which represents anaverage of five tests.

EXAMPLE II In another example, a 300 pound commercial heat of the alloywas made in an induction furnace. The alloy (Alloy No. 3) containedabout 21.2% nickel, about 3.75% manganese, about 2.48% carbon, about1.75% silicon, about 0.019% phosphorus and about 0.032% magnesium. Anumber of castings including bars about 3 inches thick were made. Thecastings contained good spheroidal graphite. The bars were normalizedfrom about 1700 F. Metal from these bars was subjected to a tensile testat room temperature and to Charpy V-notch impact tests at varioustemperatures from room temperature down to 423 F. with the results shownin the following Tables IV and V.

CHARPY V-NOTCH IMPACT TESTS Impact strength 1 Temp., F.: foot poundsRoom 24 Average of two tests except for value at 423 F. which representsaverage of four tests. In addition, castings made of the alloy wereunbroken in the Naval Research Laboratory drop weight test at 320 F.,thus indicating that the nil ductility temperature (NDT) for the alloywas below 320 F. The aforementioned drop Weight test is described in theliterature, for example, in The Welding Journal, volume 33, No. 9,Research Supplement, page 481s et seq. (1954), and in The WeldingJournal, volume 38, No. 5, Research Supplement, page 209s et seq.(1959).

It will be seen from the foregoing that the special austenitic ductileiron composition produced in accordance with the present inventionprovides very high ductility at room temperature together with hightensile strength at room temperature. Thus, the alloy will exhibit aroom temperature tensile elongation in the normalized condition of atleast about 35%, and usually at least about 40%, together with a tensilestrength of at least about 65,000 psi. The alloy will have a very lownil ductility temperature which will usually be below 423 F. Inaddition, the alloy has high impact resistance at room temperature andthis high impact resistance is retained to a marked degree even attemperatures as low as 420 F. For example, the alloy is characteriZed byan impact resistance as measured by the Charpy V-notch impact test of atleast about 15 foot pounds at 320 F. When the nickel content of thealloy is at least about 21%, the manganese content is at least about3.7% and the carbon content is not more than about 2.6%, the alloy willdisplay a Charpy V-notch impact value of at least about 20 foot poundsat 320 F. and of at least 15 foot pounds at -423 F. Furthermore, thealloy does not exhibit thermal martensite even down to temperatures aslow as -423 F. Thus, metal from Alloy No. 1 was cycled between roomtemperature and 320 F. for 20 cycles and this treatment did not produceany martensite in the alloy.

The special and distinctive combination of properties characterizingalloys within the invention enable the use of the alloy in castings atthe very low temperatures encountered in cryogenic service. Thus, pumpcastings, pump impellers, valves, fittings, compressor components, etc.,may be produced as castings from the alloy provided in accordance withthe present invention.

The alloy produced in accordance with the present invention may bewelded using any of the standard techniques employed for Welding castiron. Thus, the stickelectrode arc, oxyacetylene, inert-arc and heli-arcmethods may be employed in welding the alloy.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. An austenitic ductile iron consisting essentially of about 2% to 3%carbon, about 1% to 3% silicon, about 20% to 24% nickel, with thecarbon, silicon and nickel contents being so related that the sum of thecarbon content plus 0.2 times the silicon content plus 0.06 times thenickel content does not exceed 4.4, about 3.25% to 5% manganese, a smallamount up to about 0.12% of magnesium effective to induce the occurrenceof spheroidal graphite in the cast iron and the balance essen tiallyiron.

2. An austenitic ductile iron consisting essentially of about 2% toabout 2.6% carbon, about 1% to about 3% silicon, about 21% to about 24%nickel, with the carbon, silicon and nickel contents being so relatedthat the sum of the carbon content plus 0.2 times the silicon contentplus 0.06 times the nickel content does not exceed 4.4, about 3.7% toabout 5% manganese, a small amount up to about 0.12% magnesium effectiveto induce the occurrence of spheroidal graphite in cast iron and thebalance essentially iron.

3. An austenitic ductile iron consisting essentially of about 20%nickel, about 3.55% manganese, about 2.61% carbon, about 2.04% silicon,about 0.07% magnesium and the balance essentially iron.

4. An austenitic ductile iron consisting essentially of about 21.2%nickel, about 3.75% manganese, about 2.48% carbon, about 1.75% silicon,about 0.032% magnesium and the balance essentially iron.

5. A11 austenitic ductile iron consisting essentially of about 21.7%nickel, about 3.86% manganese, about 2.58% carbon, about 1.9% silicon,about 0.031% magnesium and the balance essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS2,842,437 Guenzi July 8, 1958

1. AN AUSTENITIC DUCTILE IRON CONSISTING ESSENTIALLY OF ABOUT 2% TO 3%CARBON, ABOUT 1% TO 3% SILICON, ABOUT 20% TO 24% NICKEL WITH THE CARBON,SILICON AND NICKEL CONTENTS BEING SO RELATED THAT THE SUM OF THE CARBONCONTENT PLUS 0.2 TIMES THE SILICON CONTENT PLUS 0.06 TIMES THE NICKELCONTENT DOES NOT EXCEED 4,4 ABOUT 3.25% TO 5% MANGANESE, A SMALL AMOUNTUP TO ABOUT 0.12% OF MAGNESIUM EFFECTIVE TO INDUCE THE OCCURRENCE OFSPHEROIDAL GRAPHITE IN THE CAST IRON AND THE BALANCE ESSENTIALLY IRON.